1. Field of the Invention
The present invention relates to methods of recovering methane from solid carbonaceous subterranean formations and, more particularly, to methods of increasing the areal and/or vertical sweep efficiency of an injected desorbing gas.
2. Setting of the Invention
In recovering methane from solid carbonaceous material it is known to inject a gas or liquid into the subterranean formation to assist in desorbing methane from the solid carbonaceous material and to move the desorbed methane towards production wells. Ensuring that the injected gas or liquid contacts the solid carbonaceous material to the areal and/or vertical extent desired is very difficult. The volume of methane recovered can be increased if the injected gas or liquid can be directed towards certain areas and away from other areas in the subterranean formation. Injected gases or liquids pass preferentially through areas of higher permeability with subterranean formation, thereby leaving recoverable methane in areas of relatively lower permeability. This preferential passage is especially true in coal seams where the natural fractures in coal provide channels or relatively high permeability areas that are generally aligned or oriented in a single direction, so any injected gas or liquid passes relatively rapidly in that single direction with little to no contact with surrounding portions of the subterranean formation.
To assist the injected gas or liquid in passing through the areas of relatively lower permeability, various techniques have been used, including drilling a horizontal wellbore generally perpendicular to the aligned natural fractures in coal. One technique used as part of a hydraulic fracturing process is disclosed in Mazza, et al. U.S. Pat. No. 4,283,089, where CO.sub.2 is injected into the coal seam to cause the coal to swell, thereby reducing the permeability of the coal adjacent the wellbore. Later, hydraulic fracturing fluid will pass preferentially into areas of the coal that have higher permeability than those areas of the coal that swelled.